Fire Resistance of Historic Fabric

Peter Jackman

The implementation of fire safety
regulations has taken its toll on historic
building fabric over the last half-century
as enforcers have tried to impose the
prescriptive recommendations given in the
guidance published in support of national
regulations such as Approved Document B
(Fire Safety). These recommendations are
frequently imposed without due consideration
of the need for them.

Elements of construction have to be
‘certificated’ as being capable of providing fire
resistance for the pre-determined periods
stated in prescriptive guidance documents.
These periods are generally stated as 20, 30, 60
or 120 minutes when evaluated against the fire
exposure conditions given in the appropriate
standards, normally BS 476: Part 20: 1987 or its
predecessors. The standards are based on fixed
pass and fail criteria relating to structural
load-bearing capacity, integrity (flaming)
and temperature rise on the protected side
(insulation) of the element.

The objectives behind these
predetermined durations are never questioned,
nor do we challenge the assessment criteria
used, such as the fire severity replicated in
the furnace, or the failure criteria used to
establish integrity (the resistance to flames
passing through the element) and insulation.
In truth, these questions do need to be asked.
Unlike new construction, where materials and
fittings are supplied with no knowledge of the
conditions in which they will ultimately be
used, we can define exactly what we want a
particular element of a particular building to
achieve in the context of reducing the impact
of fire. Once this has been defined, we are in
a position to ascertain whether the element in
question is capable of fulfilling that role either
with or without the application of upgrading
measures.

While ultimately this may require the
assistance of a professional, initially an
analysis can be made by the interested parties
following the principles outlined here, which
will lead to an informed approach to the
specification of works.

The first myth that must be addressed
is the time element. Practitioners are often
shocked to learn that fire resistance (or FR)
minutes as referred to in the guidance, have
no correlation with real time minutes. The
actual correlation between FR minutes and
real minutes is not only an unknown, it
varies building by building, structure by
structure. This statement is confirmed in
ISO/PD TR 834-3, the International Standard
Organisation’s fire resistance test method
commentary. ISO 834 forms the basis of
BS 476: Part 20, which explicitly identifies
this lack of correlation between FR and real
time minutes. In a situation with a plentiful
supply of combustible material and a good
draught an element with a 30-minute fire
resistance might provide just 10 minutes
fire separation, while in another situation
it might provide 45 minutes. Nothing
worries fire engineers more than seeing
less experienced designers halving the
fire resistance period on the grounds that
‘everybody will escape in under five minutes’.
This is acceptable if they are halving an
application where the 30 minutes FR provides
a real 45 minutes, but where the actual
duration is only 15 minutes, this could reduce
the protection afforded to 7½ minutes.

What features of the building determine
whether 30 FR minutes result in an under or
over-provision in practice? There is no easy
answer to this, but a major influence is the
likely severity of the ‘real’ fire and the way
the fabric responds to these conditions. A new
fire standard, BS 9999, gives a clue as to how
this is determined. This standard generates
fire safety measures that relate to an alpha-numeric risk profile which is the product of the building’s use and contents. A major
component in this risk profile is the severity/rate of fire growth, which has four categories:
slow, medium, fast and ultra-fast.

A slow-growing fire will take much longer
to damage the fabric, or generate untenable
conditions on one or both sides of a separating
element, than a fire that is growing ultra-fast.
The furnace used to measure fire resistance
displays a fixed fire growth rate, probably
‘fast’ for the first 10-15 minutes and ‘medium’
thereafter. A construction which achieves
30 FR minutes against the furnace curve will
obviously last longer when subjected to a slow
growing fire and vice versa.

A corridor lined with drapes with a low integrity tolerance

What factors control fire growth? The
fire load is important; a fire in an art gallery
generally has significantly less fuel than one in a
library. However, this simple analysis is also
dependent upon the amount of air available
for combustion. An unfenestrated basement
full of books would probably have a slower
growth rate than a modestly furnished large
dining hall, despite the disparity in the fire
load. The fabric of the construction will then
influence the fire conditions. Unplastered
stone walls will produce a lower rate of heating
than a hollow lath and plaster wall. Clearly,
this analysis is not ‘rocket science’, and a
consultant with a certain empathy for the likely fire dynamics will be able to predict
the appropriate rate of fire growth and likely
applicability of the furnace-generated fire
conditions with some confidence.

What then is the validity of the standard
failure criteria, particularly integrity (passage
of flame)? The obvious failure criterion is the
presence of flaming on the unexposed face.
The presence of any flame on the protected
face is irrefutable evidence of failure against
this criterion, but what is the real hazard? In
a tapestry-hung drawing room the hazard
is very real but in an unfurnished plastered
corridor, is this hazard significant before
flames reach a certain size? The test operated
in the UK for over 20 years utilises an oven
dry cotton pad placed in the vicinity of any
through-gap, even when no flames are being
generated, which indicates a loss of ‘integrity’
when it starts to smoulder. Even a tapestry-hung
room could tolerate that level of leakage
for some time and a stone-walled armoury
could withstand it almost indefinitely. Does it
always constitute failure?

The failure criteria for insulation are
deemed to have been exceeded when the
average temperature of the unexposed face
exceeds around 160°C (a rise of 140°C). What
hazard does such a temperature represent in
terms of life safety and fire spread?

In respect of life-safety, touching a wall
at 160°C would result in serious burns: this is
far from being a safe temperature. However,
in respect of fire spread, this temperature
is not going to ignite most combustibles in
the adjacent space, even in our proposed
tapestry-hung room. Ignition is only likely
if temperatures of around 400°C are reached
on the protected face and the combustibles
are in close contact. Therefore, if the standard
criterion of 160°C is used, the safety margin is
very large.

Having recognised how the exposure
conditions will vary, and how appropriate
the test failure criteria are to the hazard, the
adequacy of the existing construction can
then be established in the context of the
objective. There is a significant difference
in the acceptability of the unexposed face
conditions depending upon whether the
barrier is to provide life safety, to restrict
the degree of damage to the structure, or to
preserve as many of the artefacts as possible.
The design of the fire barrier will vary
significantly against these three objectives.

If the objective is solely life safety, (which,
as the supporting guidance indicates, is the
sole concern of the Building Regulations), it
will be necessary to provide a short period
that is primarily smoke-free and not too hot,
after which the fire can be allowed to spread
as the structure and the artefacts will be
seen to be sacrificial; the unwritten result of
adopting national Building Regulations. In
order to prevent the main structure from
being ravaged by fire, we would not only wish
to keep the occupants alive, we also try to
contain the fire for as long as possible, but
not being overly-concerned about allowing
more smoke/steam or elevated temperatures
to develop. When the artefacts have historical
or monetary value, then our protection levels
need to be higher, controlling smoke and
temperature rise for much longer and so
allowing snatch teams to do their job safely.

Once an understanding of the fire growth,
the objectives and the relevant criteria
has been achieved, it becomes possible
to establish the adequacy of the existing
construction by bringing in professional
help. For many applications, little physical
upgrading may be necessary: it may simply
be a case of making good the existing fabric
as required. If inadequate, the upgrading
measures can be minimised by focussing
solely on any shortfall in performance.

LATH AND PLASTER CEILINGS

Lath and plaster ceilings have a major role
in preventing fire spread. They are critical to
the protection of horizontal elements such as
timber joisted floors, including the flooring
on top, which in terms of fire performance is
often in a poor condition due to the presence
of gaps. Following failure of the ceiling, fire
may spread either as a result of integrity
losses through gaps in floorboards or as a
result of collapse due to the joists burning
away. The longer the ceiling remains in place,
the greater will be the fire separation between
floor levels.

Published data on the fire resistance
of lath and plaster ceilings is limited.
Not surprisingly, it has turned out to be
impractical to remove a fully aged, installed
and ‘abused’ ceiling to a fire test laboratory
for testing without causing unknown
degrees of damage. Ceilings built in place
in test laboratories are rarely adequately
aged, although age is known to have a major
influence on the fire resistance of lime
plasters. This is because lime cannot achieve
its maximum strength and hence full fire
potential until it has fully carbonated, and
this can take many months.

The testing that has been undertaken
as part of post-war building studies on
lath and plaster ceilings has revealed wide
variations in performance when tested under
the standardised furnace conditions, with
some exhibiting collapse quite quickly. The
poor performance can, it is thought, be
due to the very rapid heating rate (thermal
shock) and the high temperatures reached
in a standard furnace test. The furnace test
is artificial because it ignores any preheating
which would be experienced in
the period before ‘flashover’, the point of
sudden escalation. Furthermore, in a less
well furnished room the rate of heating will be slower, with restricted fenestration slowing it further. When the
ceiling is heated at this much slower rate, the ceiling may be adequate
for some applications without any need to upgrade the ceiling at all.
Early testing of an American intumescent coating system is already
indicating that the rate of plaster heating can be reduced by a brush/
roller applied coating, significantly increasing the duration of its
integrity.

Whether the extended protection is needed will depend upon the
objectives set for the protected space. An expert analysis of any floor/ceiling assembly, measured against the set objectives may conclude
that, once any repairs required have been carried out, it is:

adequate without treatment

adequate with, for example, a brush/roller applied coating to the ceiling

not adequate without additional linings or floor cavity
insulation being incorporated, although reducing the
fire load and/or reducing the ventilation may permit
a positive re-analysis without such measures.

FIRE DOORS

A similar approach can be applied to fire doors. The performance of a
door that is required to provide fire separation will also be a product of
the fire severity and the rate of heating, which are both related to the
quantity of combustible material (the fire load) and its vulnerability.
Whether the door is adequate for the required purpose will depend
on whether the objective is life-safety only, property protection or
protection of the contents.

Typical door assembly that could easily be upgraded to
provide adequate fire resistance in a low to medium rate
of fire growth application

To achieve life-safety, one of the most important measures will be
the fitting of a non-invasive smoke seal and a face-fixed high quality
intumescent seal.

When the objective is extended to containing the fire to protect the
structure and/or the artefacts, the door must prevent burn-through
for more prolonged periods. In a panelled door, this will invariably
mean that the panel must be upgraded but, above all, the method of
retaining the panel must be improved. This will probably require a
contribution from a fire-door expert. Painting the whole of the door
with intumescent paint is highly questionable, as the main framework
of the door will rarely burn through. Upgrading the panel is most
important, but any treatment of the panel should utilise proven
applied membranes such as thin sheets of intumescent mill-board and
not just rely on a coat of intumescent paint or a layer of intumescent
paper. Coatings may, however, slow down the rate of heating.

Fire doors almost exclusively fail as a result of the leaf distorting
out of the frame, rather than by burning-through; the result of
asymmetric heating. Again, this distortion can also be the product of
rapid heating, so if the potential fire development is slowed down by
reducing the contents and controlling the air supply, then the door
will remain adequate for longer without the need for major work on
the leaf. While this process may sound complicated, it is possible for a
specialist fire consultant with experience in dealing with historic fabric
to apply an expert system that utilises these principles and generates
the minimum number of upgrading measures.

MINIMUM INTERVENTION, MAXIMUM EFFECT

These examples demonstrate that, with a review of the fire resistance
objectives and an analysis of the environment in which the building
exists, it is possible to dispense with a great deal of unwanted ‘surgery’
without compromising the objectives. These principles can be used
in relation to any element of a building, allowing a scientific common
sense approach to the upgrading process.

Ultimately, as with modern hospital surgery, the process of
upgrading our historic buildings for fire safety should never be
unnecessarily invasive. Rather, it should be a case of making the
minimum of intervention for the maximum effect.

The Building Conservation Directory, 2009

Author

PETER JACKMAN is the chairman and technical director of International
Fire Consultants Ltd and the IFC Group. He was lead author of the
BS476 fire test standard and a major contributor to the European
(EN) and International (ISO) equivalent standards. IFC has developed
an expert system for the assessment of the criteria affecting fire
separation requirements in historic properties which is increasingly
being used to avoid unnecessary upgrading.